1. Field of the Invention
This invention relates generally to a braking assembly for stopping the rotation of a shaft and, more particularly, to an emergency braking assembly for stopping the rotation of a shaft if the speed of the shaft exceeds a predetermined speed, where the braking assembly employs weights that slide outward under centrifugal force and cause pins to engage a rotating braking ring.
2. Discussion of the Related Art
Many critical operations in elevating, hoisting, material handling and transfer operations require the use of braking mechanisms to prevent fatalities, injury and damage to property. Typical examples include elevators, man lifts, hoists, locks, conveyers and other machinery designed to overpower gravity, water or wind energy. Failure of these braking mechanisms can lead to a potential runaway condition that can be avoided by the use of redundant braking mechanisms.
Existing systems designed to provide redundant braking are typically powered or released by the same power source that the machine uses, such as electricity, pneumatic pressure or hydraulic pressure. This means that the reliability of these redundant braking systems can only be as reliable as the power source itself.
One application where emergency braking is critical is for overhead hoists and winches. An overhead hoist or winch typically lifts and lowers a load by using a motor that winds and unwinds a cable onto a cylindrical cable drum rotating upon a shaft. The motor includes an output shaft that is coupled to a gear box having an output shaft. The gear box output shaft is coupled to the cable drum shaft so that the gears control the speed and power provided by the motor to the cable drum.
In a typical hoist or winch design, a braking device is attached to the motor. In one design, the braking device includes a spring loaded plate that applies a force against a stationary plate. When the motor shaft is to be rotated, an electrical signal controls an electromagnetic coil that releases the spring loaded plate against the bias of the springs to allow the motor shaft to rotate. Thus, if power is not available to operate the motor, the springs that maintain the motor in the locked position prevent it from rotating.
If the motor brake is not periodically inspected and maintained, contamination and normal wear can affect the ability of the braking device to prevent the motor shaft from rotating. Thus, if the hoist or winch is carrying a load and power is disrupted, the motor brake may not be able to prevent the load from falling. Further, these types of hoists and winches typically do not include a braking device downstream of the motor. Particularly, failure of any of the several gears in the gear box may allow the cable drum shaft to rotate independently of the rotation of the motor shaft. Thus, a potential hazardous condition exists where a gear box failure could cause a suspended load on the hoist or winch to drop, possibly causing injury or worse.